The present invention places itself in the field of the electric machines with a discoidal structure. It relates more particularly to a method of and a device for the compensation for magnetic pull forces between the rotor and the stator of such a machine.
The electric machines with a discoidal structure comprise at least one disc-shaped stator held on a casing and the central portion of which is generally hollowed out as well as a rotor also in the shape of a disc mounted onto a shaft the axis of which coincides with that of the stator disc.
One may in particular cite the patent FR-2,583,231 which describes a discoidal machine in which the rotor disc is placed between two stator discs, the windings of the rotor and stator discs being placed on confronting faces.
One may also contemplate a discoidal machine comprising a stator disc with a double face placed between both discs of the rotor.
In both cases the faces of the stator and of the rotor are separated by air gaps. At the time of the mounting, the dimensions of these air gaps are provided to be approximatively equal. In this type of machine, the rotor may exhibit axial offsets with respect to the stator in view of the mechanical machining and mounting tolerances of the different elements. One also finds that the dimensions may vary owing to the thermal expansions of the materials and mechanical forces.
The flux which flows through the poles of the discoidal machine comprises two components: the main flux and the secondary flux also called interpolar flux.
The main flux axially extends through each disc. The generated forces therefore have opposite directions and the resultant of these forces theoretically is zero.
The inteirpolar flux comprises variable components as a function of the number of poles and of the dimensions of the air gap. It generates forces which in theory counterbalance one another.
However due to the fact of the dimensional variations appearing inside of the discoidal machine, the forces generated by the interpolar fluxes at the level of each air gap do not counterbalance one another in practice.
Thus during the operation of the machine, substantial axial forces appear on the rotor and the stator, these forces being due to the differences of inteipolar flux on each face of the rotor or stator.
These forces may result in the deterioration of the discoidal machine.